Logging method and apparatus for measuring earth formation resistivity as well as arm mechanism for the same

ABSTRACT

In a well logging sonde having a sonde body and wall engaging pad, the sonde body comprises two conductive sections spaced from one another by an insulating section. The pad and sonde are maintained in a relative longitudinal position such that the focussing effect from the upper sonde section is relatively weak or negligible for current emitted from the lower pad face and relatively strong for current emitted from the upper pad face. The current is returned through the lower sonde section. Additionally, the current electrodes are provided with particular individual characteristics that are defined to optimize the signal-to-noise ratio of the respective measurements. The pad may be provided with voltage electrodes on a lower insulating region of its face to allow for correction of the measurements for the resistivity of the borehole mud. The pad may be provided with additional current electrodes by which the application of the pad to the borehole wall may be monitored.

The subject matter of this application is related to subject matterdisclosed in the following copending U.S. patent applications, eachfiled on even date herewith and assigned to a common assignee: U.S.application Ser. No. 511,412 of H. Ohmer, and U.S. application Ser. No.511,414 of S. Locke and M. Gouilloud.

BACKGROUND

The present invention relates generally to the measurement of earthformation resistivity from a borehole traversing the formation, and morespecifically to the measurement of the resistivity and/or thickness ofone or more regions about the borehole.

In many conventional drilling operations, the mud in the borehole isconditioned so that the hydrostatic pressure of the mud column isgreater than the fluid pressure of the formations. The effect of thiscondition is represented in FIG. 1, which shows an exemplary radialdistribution of resistivities in a water-bearing bed.

The differential pressure forces mud filtrate into permeable formationstraversed by the borehole, whereby the solid particles of the mud aredeposited on the borehole wall to form a mudcake of resistivity R_(mc).This mudcake usually has very low permeability and considerably reducesthe rate of infiltration as it builds. In the portion of the formationsimmediately about the borehole, however, all of the formation water andsome of the hydrocarbons, if present, are flushed away by the filtrate.This zone is called the flushed zone, and its resistivity is expressedas R_(XO). Farther out from the borehole the displacement of formationfluids is less and less complete, resulting in a transition zone inwhich a progressive change in resistivity occurs from R_(XO) to theresistivity of the uninvaded formation, R_(t). The lateral depth ofinvasion depends in part on formation permeability and is quitevariable, ranging from less than one centimeter to several tens ofcentimeters.

Measurements of R_(XO) are important for several reasons. When invasionis moderate to deep, knowledge of the R_(XO) value makes possible moreaccurate determinations of true resistivity R_(t), which is related tohydrocarbon saturation. Also, some methods for computing saturation areentered with the ratio R_(XO) /R_(t). Also, in clean formations, a valueof the Formation Factor F may be computed from R_(XO) and theresistivity of the mud filtrate "R_(mf)," if the mud filtrate saturation"S_(XO) " is known or estimated. From F, a value for porosity may befound. A more recent application for R_(XO) data is in conjunction witha full evaluation of hydrocarbon effects on the Neutron and Densitylogs, an integral part of the SARABAND™ and CORIBAND™ services availablethrough Schlumberger Technology Corporation, Houston, Tex. Moreover, thecomparison of R_(XO) and R_(t) is useful for understanding hydrocarbonmoveability.

In addition to recognizing the usefulness of the R_(XO) measurement, theprior art has been concerned with acquiring information that isindicative of the locations of lateral discontinuities in electricalresistivities of borehole surrounding materials to determine wellborehole diameter and depth of fluid invasion. U.S. Pat. No. 2,754,475(stated inventor: Norelius; issued July 10, 1956), for example,discloses that resistivity measurements may be made at continuouslyvarying lateral depths in the borehole and formation surrounding theborehole by measuring throughout a plurality of borehole locations theaverage resistivity of formation and borehole fluid contained withinrespective spherical shells. The shells are established by an electricalfield that converges substantially radially through surroundingformations upon an input electrode in the fluid within the wellborehole. The borehole radius, depth of invasion, and the location ofthe uninvaded formation can be determined by identifying anydiscontinuities which appear in a display.

It has been recognized in the art that a wall-engaging pad device ismost appropriate for the measurement of shallow invasion, however. Manysuch devices have been proposed, one of which is disclosed in U.S. Pat.No. 2,669,688 (Doll, Feb. 16, 1954). One of the embodiments described inthe Doll '688 patent includes a three electrode system comprising acurrent electrode and two potential measuring electrodes, with thecurrent return being on the cable. Measurements made with this systemare indicative of the resistivities at different shallow lateral depthsof investigation. One depth of investigation is approximately equal tothe probable thickness of mudcake on the wall of the borehole, and theother is slightly greater so as to include the mudcake and at least apart of the adjacent portion of the formation that has been invaded bythe mud filtrate. Since the presence of mudcake on the wall of aborehole is an indication of invasion of the formation by mud filtrate,proper interpretation of the measurements so made enables permeableformations to be identified. A third measurement at yet another shallowlateral depth of investigation also facilitates the identification ofpermeable formations. See, e.g., U.S. Pat. No. 2,965,838 (Kister, Dec.20, 1960).

To accurately measure the value of R_(XO), however, the measurement mustnot be affected by the borehole or must be capable of being corrected.Measurements made at different shallow lateral depths of investigationcan be corrected when at least one of the measurements accurately yieldsmudcake resistivity. See, e.g., the Kister patent. Borehole effects onthe R_(XO) measurements can be minimized, however, by the use offocussing currents to control the path taken by the survey current.Borehole effects, particularly severe as the mudcake thicknessincreases, arise when the survey current is shunted back to the boreholeby the relatively low resistance path formed by the mudcake so that theformation measurement is influenced to a large extent by the mudcakeresistivity. Several apparatus have been proposed to overcome thisproblem. The focussed pad system disclosed in U.S. Pat. No. 2,712,629(Doll, July 5, 1955) is particularly suitable for use where a minimum tomoderate thicknesses of mudcake occurs and in salty muds. In thefocussed pad system disclosed in U.S. Pat. No. 3,132,298 (Doll, May 5,1964), an apparatus is proposed which performs satisfactorily inrelatively thick mudcakes. More recently, pad mounted electrode toolshave been developed which provide a greater accuracy in obtainingR_(XO), especially in thick muds. This new type of well logging tool hasbeen referred to as a spherically focussed apparatus and is described inU.S. Pat. No. 3,760,260 (Schuster, Sept. 18, 1973). The sphericallyfocussed system also has been proposed to determine the lateralthickness of a mudcake as well as R_(XO). See, e.g., U.S. Pat. No.3,973,188 (Attali et al., Aug. 3, 1976). All of the above-mentionedfocussed microresistivity tools provide good R_(XO) measurements undercertain conditions, although none provide accurate R_(XO) values underall conditions.

Wall-engaging pad devices also have found application in thedetermination of dip. Dip determining devices, known in the art asdipmeters, characteristically employ four pads which are applied againstthe borehole wall through two perpendicular diameters. Typically, eachpad contains a transducer which conducts an investigation of formationcharacteristics immediately adjacent to the pad. See, e.g., U.S. Pat.No. 3,060,373 (Doll, Oct. 23, 1962). An individual pad may be providedwith more than one electrode for the purpose of improving thedemarcation between bed boundaries, see, e.g., Doll '373; enhancingdipmeter signals over noise, see, e.g., U.S. Pat. No. 3,521,154(Maricelli, July 21, 1970); removing variations in speed caused by theso called "yo-yo" effect, see id.; or allowing more detailed correlationof signal features corresponding to vertical changes in formationcharacteristics, see, e.g., U.S. Pat. No. 4,251,773 (Cailliau et al.,Feb. 17, 1981).

Dipmeter tools typically use passively focussed electrodes systems forconstraining the surveying current to penetrate laterally for anappreciable distance into the adjacent earth formation. Typically, thefocussing current electrode is a metallic surface which forms the majorportion of the pad face. Centrally located in the pad face is a recesscovered by a layer of insulating material. A survey current electrode isdisposed in the recess and is separated from the metal pad proper by theinsulating material. Survey current emitted from the survey electrode iscaused to penetrate laterally into the adjacent earth formation by thecurrent emitted from the focussing electrode. Additional focussingcurrent may be emitted from the conductive surface of the dipmeter sondebody. Where more than one survey electrode is provided, the additionalelectrodes also are completely surrounded by the focussing electrodesuch that the respective survey current beams are focussed as describedabove by current emitting from the focussing electrode. The currentreturn may be via a return electrode B located on the lower end of themulti-conductor cable as in the Doll '373 patent, or via a tool bodymember as in the Cailliau et al., patent.

In the prior art dipmeter systems, the survey current beams are focussedto penetrate relatively deeply into the earth formation in front of thepad member so that an appreciable portion of the electrical resistanceexperienced by the beam in the earth formations will be contributed bythe uncontaminated portion of the formations, even though this zone isspaced from the borehole proper by a mudcake and an invaded zone.Moreover, the focussing and depth of penetration of all survey currentstypically are essentially identical to facilitate dip determination and,in the case of a multiple electrode pad tool, to facilitate noisecancellation, speed correction, and/or more detailed correlation offeatures.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to provide signalssuitable for resistivity determinations under a greater variety ofinvasion conditions than possible in the prior art. Another object ofthe present invention is to provide signals suitable for resistivitydeterminations of greater accuracy than possible in the prior art. Yet afurther object of the present invention is to provide signals suitablefor determining the thickness and/or resistivity of one or a pluralityof zones about the borehole.

A wall-engaging pad in accordance with the present invention comprises aface portion of electrically conductive material within which aredisposed three or more current electrodes. The current electrodes andface portion of the pad are arranged such that the current emitted fromthe current electrode nearest a return electrode on the sonde body isrelatively weakly focussed or unfocussed and penetrates only a shortdistance while the current emitted from the current electrode farthestfrom the return electrode is relatively strongly focussed and penetratesa relatively great distance. Accordingly, the depth of investigationvaries along the pad from on the order of a few millimeters to 20centimeters under certain conditions. A plurality of individualmeasurements having respective graduated depths of investigation aremade with the current electrodes. These measurements may be suitablyprocessed to yield resistivity measurements.

The pad and sonde body cooperate to enhance the focussing of the surveycurrent. Advantageously, the sonde body comprises two conductivesections spaced from each other by an insulating section. The pad andthe sonde are maintained in a relative longitudinal position such thatthe focussing effect from the upper sonde section is relatively weak ornegligible for the current emitted from the lower pad section andrelatively strong for the current emitted from other sections of thepad. The current is returned through the lower sonde section.Additionally, the current electrodes may be provided with individualcharacteristics that are defined to optimize the signal-to-noise ratioof the respective measurements. Moreover, voltage electrodes may beprovided on a lower region of the pad to allow for correction of themeasurements for the mud resistivity.

Other objects, features, and characteristics of the invention will beapparent upon perusal of the following Detailed Description and Claims,with reference to the accompanying Drawings, all of which are part ofthis Specification.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference characters indicate like parts:

FIG. 1 (Prior Art) is a graph of the radial distribution ofresistivities in a water bearing bed where R_(mf) is significantlylarger than R_(w) ;

FIG. 2 is a plan view of one embodiment of a borehole logging tool inaccordance with the present invention, wherein the earth formationtraversed by the borehole is illustrated in cross-section and surfaceequipment is illustrated schematically;

FIG. 3 is a plan view of the face of one embodiment of the wall-engagingpad of FIG. 2;

FIG. 4 is a longitudinal cross-sectional view of the pad of FIG. 3,wherein associated pad electronics are shown schematically;

FIG. 5 is a graph for explaining the use of the signals produced inaccordance with the present invention;

FIG. 6 is a flow diagram for explaining the creation and use of thegraph of FIG. 5;

FIG. 7 is a flow diagram for explaining the use of signals produced byan embodiment of the present invention having more than three currentelectrodes;

FIG. 8 is a graph for explaining certain steps of the flow diagram ofFIG. 7;

FIG. 9 i is a plan view of another embodiment of a borehole logging toolin accordance with the present invention, wherein the earth formationtraversed by the borehole is illustrated in cross-section and surfaceequipment is illustrated schematically;

FIG. 10 is a plan view of an embodiment of the wall-engaging pad of FIG.9;

FIG. 11 is a longitudinal cross-sectional view of the pad of FIG. 10,wherein associated pad electronics are shown schematically;

FIG. 12 is a lateral cross-sectional view of the pad of FIG. 10;

FIG. 13 is a graph for explaining the current density on the face of thepad of in FIG. 10;

FIG. 14 is a plan view of an arm mechanism in accordance with thepresent invention for eccentering the resistivity sonde and maintainingthe pad in engagement with the borehole wall, wherein the earthformation traversed by the borehole is shown in cross-section and themechanism is shown in the pad extension mode;

FIG. 15 is a plan view of the arm mechanism of FIG. 14 shown in the padretraction mode; and

FIG. 16 is a lateral cross-sectional view of the mechanism of FIG. 15,taken along section F16--F16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of a downhole investigating tool 10 is illustrated inFIG. 2. The tool 10 is disposed in a borehole 12 through earth formation14. The borehole 12 is filled with a suitable drilling mud. The end ofthe tool 10 nearest the surface is connected by means of an armoredmulti-conductor cable 16 to suitable apparatus at the surface of theearth for raising and lowering the tool 10 through the borehole 12. Themulti-conductor cable 16 passes over a sheave 18 and then to a suitabledrum and wench mechanism 20. Electrical connection between the variousconductors of the multi-conductor cable 16 and telemetry, control andpower circuits 24 at the surface of the earth is accomplished by meansof cable 22 and a suitable multi-element slip ring and brush contactassembly integral with the drum and wench mechanism 20. Cable depthinformation is provided by measurement wheel 26 and supplied via cable28 to the telemetry, control and power circuits 24. Signals from tool 10containing information on a characteristic of formation 14 are suppliedto processor 26 from the telemetry, control and power circuits 24. A logof the formation characteristic versus depth is produced by processor 26and furnished to display device 28 for viewing by a user.

Resistivity Sonde and Pad Embodiments

The tool 10 comprises a number of individual sections, including severalcartridges, indicated generally at 30, for supplying power to the tool10 and controlling its operation, for preprocessing measurement signals,and for providing a telemetry interface between the downhole electronicsand the cable 16; and a sonde indicated generally at 32 for providingmeasurement signals in accordance with the present invention. The sonde32 comprises three longitudinally contiguous elongated sections 34, 36and 38. Sections 34 and 38 are made of electrically conductive metalsuch as stainless steel. The section 36, which is disposed betweenconductive sonde sections 34 and 38, performs an insulating function.One suitable arrangement, which is described in the aforementioned Doll'373 patent, includes a sleeve of epoxy or viton resin disposed aboutprojecting portions of sections 34 and 38, which are insulatinglycoupled to one another. The section 34 further supports arm 40 to whicha wall-engaging pad or skid 42 is connected, and a backup arm 44. Backuparm 44 and arm 40 cooperate preferably although not necessarily toeccenter sonde 32 in the borehole 12. The sonde 32 and pad 42 aremaintained substantially about a plane of symmetry passing through thecenter of each by arm 10. Two friction shoes 46 and 48, respectivelyrigidly secured to sections 34 and 38 and angularly aligned togetherwith pad 42 relative to the central axis of sonde 32 are provided toprevent contact of the sonde 32 with the wall of the borehole 12. Shoes46 and 48 are suitably made of low friction material such as bronze toreduce friction during vertical displacement of tool 10 in borehole 12during a logging operation.

A practical implementation of pad 42 is shown in detail in FIG. 3, whichpresents a frontal view of the pad 42, and FIG. 4, which presents across-sectional view of the pad 42 and associated electronics(schematically illustrated). The pad 42 is elongated in the direction ofmovement and its face is curved to match the curvature of the wall ofborehole 12. The conically tapered upper face section 50 allows pad 42to ride the wall of borehole 12 as the tool 10 traverses the borehole 12toward the surface during a logging operation. An upper face section 54along with the leading edge 50 of pad 42 function as a large focussingelectrode. Pad face sections 50 and 54 are made of a good conductingmaterial, such as bronze or soft iron, and are coupled to padelectronics 74 by cable 92. Within the face section 54 arelongitudinally disposed three current electrodes 56a, 56b and 56c, eachof which is mounted flush to face section 54 and is supported andelectrically insulated from the face section 54 by respective insulatingsupports 62, 64 and 66. The current electrodes 56a, 56b and 56c, whichmay be made of stainless steel, are embedded in an electricallyinsulating material such as Araldite, for example, as shown at 76, 78and 80. The current electrodes 56a, 56 b and 56c are coupled to padelectronics 74 by respective electrical cables 68, 70 and 72. A lowerface section 82 of the pad 42 is made of an insulating material such ashard rubber or epoxy resin. Centrally disposed within the face section82 is a return electrode 84, which is supported by a insulating support86 made of a ceramic material and is embedded in an insulating material88 such as Araldite. The return electrode 84 is connected to electronics74 by cable 90. A conically tapered lower face section 52 trails pad 42as the tool 10 traverses borehole 12 during a logging operation.

Typical dimensions for sonde 32 are as follows. Sonde sections 34, 36and 38 respectively longitudinally measure 100 cm (39.37 in), 20 cm (7.9in), and 100 cm (39.37 in). Sonde 32 is 10 cm (3.9 in) in diameter andnominally stands off the wall of borehole 12 by 5 cm (2 in) in the planedefined by the central axes of sonde 32 and pad 42. Typical dimensionsfor pad 42 are as follows. The current electrodes 56a-56c and 84 are 0.5cm in diameter. Current electrodes 56a-56c are longitudinally spacedfrom the return electrode 84 by distances L_(a) =6.5 cm, L_(b) =9.5 cm,and L_(c) =12.5 cm. Measured longitudinally, face sections 54 and 50 are13 cm and face sections 82 and 52 are 13 cm. For a typical 8 inchborehole, face sections 54 and 82 have a characteristic radius of 4.5inches with a lateral extension of ±25°.

Electronics suitable for operation of the pad 42 in association withsonde 32 are shown generally at 74 in FIG. 4. Signal generator 94supplies electric current to the pad 42, and more specifically to theface section 54 and to the individual current electrodes 56a, 56b and56c through respective measurement signal circuits 96a, 96b and 96c. Thecurrent is returned to signal generator 94 via the return electrode 84.The frequency of the current provided is 1,010 Hz.

Measurement signal circuit 96c is shown in detail in FIG. 4, measurementsignal circuits 96b and 96c being substantially identical in design andgenerally represented thereby. Survey current from the signal generator94 is provided to current electrode 56c through input transformer 98.Transformer 98 couples the signal to a variable-gain isolationpreamplifier 100, whose gain and scaling parameters are controlled fromthe surface. The output from the variable-gain isolation preamplifier100 is provided to a phase detector 102, which also receives the signalgenerated by signal generator 94. Phase detector 102 measures only thepart of the measurement signal which is in-phase with the currentsupplied by the signal generator 94. The output from the phase detector102 is sent to low-pass filter 104, which integrates the signal andproduces a signal whose amplitude is characteristic of the amplitude ofthe detected measurement signal. This integrated measurement signal isprovided to ratio circuit 106, which also receives the signal fromsignal generator 94 that has been integrated by integrator 108. Theoutput from ratio circuit 106, which is indicative of an apparentresistivity as measured by the current electrode 56 c, is amplified inamplifier 108 and provided to the surface as signal R_(c). The inputtransformer 98 and preamplifier 100 typically are mounted on the backside of the associated pad. The rest of the electronics of themeasurement circuit 96c may be contained in the electronics cartridge intool 10, in the sonde 32, or mounted on the back of the pad 42.

Current from signal generator 94 also is provided to the sonde section34 for improving the focussing, as described below. The current returnto signal generator 94 is shown connected to sonde section 38, but theconnection may be omitted without seriously degrading the measurement.

The operating principle of the sonde 32 is illustrated schematically inthe right hand part of FIG. 2. The current emittted from the currentelectrodes 56a-56c and from the pad wall-engaging face section 54 andtapered face section 50 penetrates into the mud, mudcake, and/orformation in returning to the signal generator 94 through the returnelectrode 84 and sonde section 38. A current density distribution isestablished on face section 54, and hence current paths from the variousportions of the face section 54 are established, which depend on thethickness and resistance of the mudcake and the invaded zones, and theresistance of the true formation. Current electrodes 56a, 56b and 56cadvantageously are positioned in areas of face section 54 havingprogressively weakening current densities, which are associated withprogressively greater depth of investigation. While the currentelectrodes 56a, 56b and 56c are arranged at regular intervals along thelongitudinal center axis of the face section 54, neither condition isnecessary, as will be made apparent below. The currents emitted from thecurrent electrodes 56a-56bc are monitored and returned to the surface asapparent resistances R_(a), R_(b) and R_(c) respectively. The currentsfrom the current electrodes 56a-56c are focussed from current emittedfrom face sections 50 and 54, and also from the sonde section 34 whichconstrains the survey currents essentially to a plane passing throughthe central axes of the sonde 32 and the pad 42. This particulararrangement advantageously minimizes beam spreading and improvesfocussing of the survey currents. Focussing is further improved bymaintaining the body of sonde 32 within a small and esentially constantdistance from pad 42 by rendering the sonde 32 eccentric, as shown.

A graphical solution for the R_(XO) value is illustrated in FIG. 5,which shows a cyclone chart having as its abscissa the ratio R_(b)/R_(a) and as its ordinate the ratio R_(c) /R_(b). The intersection ofrespective values for these two ratios yields values for h_(XO) andR_(t) /R_(XO). The values of R_(t) /R_(XO) are shown paranthetically inFIG. 5 and run left to right from 2 to 200. The values of h_(XO) runright to left from 0.5 to 8.0. For example, where sonde 32 indicates aR_(b) /R_(a) ratio of 3.5 and a R_(c) /R_(b) ratio of 2.5, the graph ofFIG. 5 yields a mudcake thickness of 2.0 cm and an R_(t) /R_(XO) ratioof 20. If the apparent deep resistivity is known, as by measurement withthe deep-focussed electrical tool disclosed in U.S. Pat. No. 3,772,589(Scholberg, Nov. 13, 1973) or other suitable electrical or inductiontool, an approximate value of R_(XO) results in straightforward fashion.A precise R_(XO) and R_(t) thereafter may be determined iteratively.

A procedure similar to that described above for forming and using thegraph of FIG. 5 can be implemented in the form of an automatic tablelook-up scheme incorporated in the processor 26 of FIG. 2. The processor26 may comprise, for example, any suitable general purpose digitalcomputer. A simplified flow chart for programming the processor 26 tocreate, store and use a suitable look-up table is shown in FIG. 6.

The inital step 110 is to suitably define the problem, i.e., define thesonde and formation. The sonde 32 is conceived as a low frequencydevice. Its mathematical representation is a boundary value problem inpotential theory, which is a class of classical mathematical problems.The sonde 32 is conceived as a mandrel which is assumed centered on therotational axis of a cylindrical coordinate system. Axially, the middleof the sonde is insulating (section 36) while the end sectionscontiguous thereto (sections 34 and 38) are conductive. Sonde 32 issurrounded by borehole fluid, i.e., drilling mud, which in turn issurrounded by concentric annuli of mudcake, an invaded zone, andundisturbed formation. The body of sonde 32 has a 5 cm radius, sondesections 34 and 38 each are 100 cm long, and sonde section 36 is 20 cmlong. The pad 42 is conceived as having an insulating midsection andconductive end sections. Pad 42 is urged against the wall of borehole12. Sonde 32 and pad 42 are arranged symmetrically about a horizontalplane perpendicular to sonde 32 and passing through the midpoint ofsonde 32 and pad 42. The length of pad 42 is selected to be 32 cm. Theconductive faces of pad 42 include electrodes 0.5 cm wide and 3 cmapart, the electrodes of interest being the three electrodes on eachconductive face nearest to the insulating section. The upper and lowersonde sections 34 and 38 are electrically connected to the upper andlower sections of the pad 42. This definition is satisfactory torepresent an arrangement wherein either the pad lower section or thesonde lower section 38 serves as the current return, or the elongatedcurrent return is collapsed into a single return current electrode.

A set of values for h_(mc), h_(XO), R_(mc), R_(XO) and R_(t) areselected in step 112, and synthetic ratios R_(b) /R_(a) and R_(c) /R_(b)are determined in step 114 by applying the finite element method, a wellknown numerical technique, to the value boundary problem. The values ofh_(XO) and R_(t) /R_(XO) are stored in memory as a function of thesynthetic ratios R_(b) /R_(a) and R_(c) /R_(b) in step 116. In step 118,a check is made to determine if a sufficient number of values for the hand R parameters have been selected to enable creation of a graph ofdesired resolution. If this has not been accomplished, a new discreteset of values for h_(mc), h_(XO), R_(mc), R_(XO) and R_(t) is selectedin step 112, and steps 114, 116 and 118 again are executed. If asufficient number of values has been selected, the look-up table isconsidered complete. Then, a measurement of R_(t) (resistivity of theuninvaded formation as measured by a deep investigation tool) andmeasurements of R_(a), R_(b) and R_(c) are effected in steps 120 and122. The processor 26 forms the ratios R_(b) /R_(a) and R_(c) /R_(b) instep 124, and consults the look-up table to find the correspondingvalues of h_(XO) and R_(t) /R_(XO) in step 126. If these values are notexactly obtainable, the processor 26 interpolates to determine thevalues of h_(XO) and R_(t) /R_(XO). Once a value for R_(t) /R_(XO) isobtained, the value of R_(XO) is determined in straightforward fashionin step 128.

The present invention also contemplates the use of more than threecurrent electrodes for the purpose of providing either a more accurateR_(XO) determination or for providing an invasion resistivity profile.An apparatus suitable for this purpose would be quite similar to theapparatus shown in FIGS. 2 and 3, except that additional currentelectrodes would be provided on the pad 42.

An alternative processing scheme for processing the measurement signalsfrom more than three current electrodes is shown in FIG. 7, where theprocessor 26 would be programmed as follows. A sonde and formationconfiguration is defined in step 140. The configuration is as set forthin FIG. 2 and associated text, except that ten electrodes are assumed.Each electrode is 0.19 inches (0.5 cm) wide and 0.4 inches (0.16 cm)apart. The overall length of the emitting section of the pad is 7.95inches. The separation between the return and the nearest emittingelectrode is 4.0 inches. The mandrel, 4.0 inches in diameter, is 64inches long and has an insulated portion 1b 18 inches long.

For purposes of illustration, a simple problem with a moderate contrastis explained. The resistivity profile, which is defined in step 142, isassumed to be step-like as shown in FIG. 8. The borehole resistivityR_(m) and the true formation resistivity R_(t) are measured with otherdevices. Three concentric annuli of assumed thicknesses h₁, h₂ andh_(XO) but of unknown resistivities R(1), R(2) and R(3), the independentvariables, are defined. The first two annuli represent either a mudcakeof two thicknesses or alternatively, standoff and mudcake. The thirdlayer represents an invaded zone, of which the thickness is the knowndepth of investigation for a particular tool. This problem representsthe practical situation of finding the R_(XO) relevant to a particulartool when there is a varying standoff or mudcake. A more complex modelrepresenting a detailed invasion resistivity profile can be defined, ifdesired.

R_(t) and R_(m) are measured in step 144 by known instruments such asthe deep investigation electrical apparatus disclosed in theaforementioned Scholberg patent and a mud resistivity measuring device,to set the value of certain parameters as described above.

The tool 10 performs a measurement of ten apparent resistivities R_(a)-R_(j) at a given depth in step 146. Initial values for R(1), R(2) andR(3) are assumed in step 148. Based on the assumed values, syntheticvalues of R_(a) -R_(j) are computed using two dimensional finite elementcode, several suitable versions of which are commercially available. Inaddition, a residual function is produced based on these assumedestimates at step 150. The residual function is a vector formed bytaking the difference, electrode by electrode, between the measuredcurrents R_(a) -R_(j) and the synthetic currents R_(a) -R_(j).

The residual function produced at step 150 is minimized at step 152 byapplication of the Levenberg-Marquardt algorithm, which is a well-knownproceedure for minimizing a function of several independent variables.The algorithm uses finite differences to determine changes to theestimates for the next iteration and to the next minimization of theresidual function. Convergence is tested in step 154. If the algorithmhas not converged, the R(1), R(2), and R(3) estimates are changed inaccordance with step 152, and step 150 is returned to for the productionof new synthetic R_(a) -R_(j) and a new residual function. Steps 150,152, 154, and 156 are iteratively executed until convergence occurs, inwhich case control is provided to step 158 which records the value ofR_(XO) and, if desired, R(1) and R(2) for the given depth.

Referring now to FIG. 9, another embodiment of the invention is shownwhich significantly increases the ratio of depth of investigation to padlength. Satisfactory depth of investigation is achieved with a shortpad, which can be easily and reliably applied to a borehole wall. Asonde indicated generally at 200 provides measurement signals inaccordance with the present invention. The sonde 200 comprises threelongitudinally contiguous elongated sections 202, 204 and 206. Sections202 and 206 are made of electrically conductive metal such as stainlesssteel. Section 204, which is disposed between conductive sonde sections202 and 206, performs an insulating function. A suitable arrangement isdiscussed above in the context of sonde section 36. The section 202supports arm 40 to which pad 210 is connected, and a backup arm 44.Backup arm 44 and arm 40 cooperate preferably to eccenter sonde 200 inthe borehole 12. Pad 210 comprises a focussing face portion 212 and aninsulating portion 214, as explained in detail below. Two friction shoes46 and 48 are provided as described above.

A particularly advantageous pad arrangement is shown in detail in FIG.10, which presents a frontal view of pad 210, and FIGS. 11 and 12, whichpresent cross-sectional views of pad 210 and associated electronics(schematically illustrated in FIG. 11 and indicated generally at 220).Pad 210 may have conically tapered sections, as described above, and iselongated in the direction of movement. The metal face 212 of pad 110 iscurved to match the curvature of the wall of borehole 12. Within theface section 212 are longitudinally disposed seven current electrodes216a-216g, which are mounted flush to face section 212 and are supportedand electrically insulated from the face section 212 by respectiveceramic elements 218abc and 218d-g. The current electrodes 216a-216g arecoupled to pad electronics 220 by respective cables 222a-222g. The facesection 212 includes two additional current electrodes 224 and 226, andthe face section 214 contains two voltage electrodes 228 and 230. Thepurpose of these electrodes is described below.

Typical dimensions for sonde 200 are as follows. Sonde sections 202, 204and 206 respectively longitudinally measure 30 inches, 15 inches, and 15inches. Sonde 200 is 4 inches in diameter and nominally stands off thewall of borehole 12 by 0.75 inches in the plane defined by the centralaxes of sonde 200 and pad 210. Typical dimensions for pad 210 are asfollows. Measured longitudinally, face sections 212 and 214 measure 8inches and 0.6 inches respectively. For a typical 8 inch borehole, facesections 212 and 214 have a characteristic radius of 4.5 inches with alateral extension of ±25°.

Preferably, pad 210 is maintained in a substantially fixed longitudinalrelationship relative to the sections 202, 204 and 206 of sonde 200.Optimal focussing is achieved if the edge of pad insulating portion 214nearest the sonde section 206 lies in a plane transversely intersectingsonde section 202 within approximately 2 cm of the juncture of sondesections 202 and 204, although this distance is not critical. A distanceof 1 cm has been selected for the embodiment of FIG. 9.

The described arrangement provides current lines having a depth ofpenetration ranging from a few millimeters to about 20 cm. This isobtained even though the length of pad 210 is small, about 20 cm.Moreover the pad 200 is highly wear-resistant as its wall-engaging faceis essentially made of metal.

The dimensions and arrangement of current electrodes 216a-216g isadapted to the distribution of current density on face section 212,which in turn is related to the depth of investigation of the surveycurrents emitted from the current electrodes 216a-216g. The curve 239shown in FIG. 13 in association with the pad 210 illustrates the densitydistribution J(z). The abscissa represents in logarithmic scale thecurrent density J(z), and the ordinate represents the longitudinalposition z on the face section 212. The graph of FIG. 13, which isexemplary, is based on R_(XO) /R_(mc) =100 and h_(mc) =0.5 inches.

Depth of investigation of the current deepens with weakening currentdensity. Depth of investigation is minimum at the lower end of the facesection 212, indicated in FIG. 13 at 234. It can be seen that from theminimum 234, the depth of investigation exhibits a rapid increase,indicated generally by 236, as the sonde section 202 and face section212 begin to be effective in focussing the current along thelongitudinal axis of pad 210. A slower increase is observed until amaximum 242 located in the upper third of the pad is reached, asindicated generally by 238. Beyond maximum 242, the proximity with theupper edge 31 causes the current density to increase and therefore thedepth of investigation to decrease, as indicated generally by 240.

The seven current electrodes 216a-216g are arranged so as to provideresistivity measurements with different depths of investigation. Thefirst electrode 216a is adjacent the insulating section 214. The secondcurrent electrode 216b is separated from the first current electrode216a by a portion of the insulating element 218abc, and the thirdcurrent electrode 216c is separated from the second current electrode216b by another portion of the insulating element 218abc. The densearrangement of current electrodes 216a-216c is adapted to the rapidvariation of the depth of investigation in section 236 of the curve 239.The three current electrodes 216a-216c are located in substantially thelower quarter of the face section 212. The three current electrodes216d-216f are spaced at greater intervals to comply with the relativelyslower variation in section 238 of the curve 239. In the section 240 ofcurve 239 including the maximum 242, which corresponds to about theupper third of the face section 212, the variation about the maximum 242is small and a single current electrode, namely electrode 216g, issufficient.

As shown in FIG. 10, the current electrodes 216b-216g have individualcharacteristics that are defined to optimize the signal-to-noise ratioof the respective measurements. Specifically the lengths and widths ofthe respective current electrodes 216a-216g are selected so as tomaintain an optional surface area to maximize the signal-to-noise ratioof the respective measurements. The respective characteristics of theelectrodes 216a-216g are based on the longitudinal variation in thecurrent density along face section 212, as illustrated by the curveJ(z), and the lateral variation of the current density across facesection 212. In the positions along the longitudinal axis of facesection 212 for which the current density is high, the current densityalso remains stable over much of the width of the pad and significantlyincreases only near the lateral edges. In the positions of low currentdensity, however, the area about the longitudinal axis of face section212 over which the current density remains substantially stable isrelatively narrow. The width of each of the current electrodes 216a-216gis defined so that the lateral variation of the current density from thecenter of the electrode (i.e., of face section 212) to the lateral endsof the electrode, is kept within a given range of, e.g., 15%. Since thecurrent density decreases with increasing distance from the section 214,the current electrodes 216b-216g have respective widths which decreasewith increasing distance from the insulating section 214. Currentelectrode 216a does not conform to this design rule for reasonsdescribed below. Thus, current electrode 216b adjacent current electrode216a has the largest width, equal to more than one-half the width of theface section 212. The current electrode 216c adjacent the currentelectrode 216b has a much smaller width, about one-half that of currentelectrode 216b. The current electrode 216d is narrower than currentelectrode 216c and so on until electrode 216g, which is the narrowest.The lengths of current electrodes 216a-216g advantageously are selectedto be as great as possible, limited by the requirement that thevariation of the current density for each electrode from onelongitudinal end thereof to the other is less than a given amount, e.g.,less than 15%. In view of the foregoing, the evolution pattern for thelength of the current electrodes 216b-216g preferably is the reverse ofthe pattern defined for the width. Current electrode 216b is shortest,and electrode length increases up to current electrode 216g, which islongest.

The current electrode 224 adjacent the upper edge of face section 212 islocated in an area of relatively strong current density and provides ameasurement corresponding to a depth of investigation less than themaximum 242. The measurement made with current electrode 224 isresponsive to the engagement of the pad with the wall of borehole 12 inthe area of the upper edge of the pad 210. Where the pad 210 stands offfrom the wall of borehole 12, current must flow through the drillingmud, causing a significant change in the apparent resistivity measuredby current electrode 224. The measurement provided by the currentelectrode 224 thus permits contact of the pad 210 with the wall ofborehole 12 to be monitored.

Current electrode 216a departs from the design rule described forcurrent electrodes 216b-216g. While current electrode 216aadvantageously may have a width nearly equal to the width of facesection 212 in accordance with the design rule, the region of facesection 212 may advantageously be used to monitor lower pad contact andprovide a measurement under poor borehole conditions. Accordingly,current electrode 226 is provided laterally of current electrode 216a,whose width is small to accomodate current electrode 226. The lateralcurrent electrode 226 provides a measurement indicative of theresistivity at a very small depth, of the order of a few millimeters,which essentially is identical with the measurement provided by currentelectrode 216a. It will be appreciated that the current density in theregion of face section 212 in which current electrodes 216a and 226 aredisposed remains stable over almost the entire width of the pad.Accordingly the current electrodes 216a and 226 should under goodborehole conditions provide quite similar measurements, even though therespective very shallow measurements are extremely sensitive to thecontact of the face section 212 with the wall of borehole 12. If one ofthe current electrodes 216a or 226 does not engage the wall of borehole12, the measurement provided by the electrode that stands off will bedominated by the resistivity R_(m) of the mud, which is usually muchsmaller than that of the mudcake and flushed zone. One of the currentelectrodes 216a and 226 will remain in contact with the wall of borehole12 under certain circumstances, however, as for example when thecurvatures of the face section 212 and the wall of borehole 12 do notagree. For example, if the curvature of the wall of borehole 12 islarger than that of the face section 212, the central area of the facesection 212 will not engage the wall. Under such circumstances thelateral current electrode 226 will engage the wall of borehole 12, and asatisfactory measurement is achieved despite the deficiency of themeasurement from the central current electrode 216a. Moreover, asignificant difference between the respective measurements of currentelectrodes 216a and 226 will be indicative of an imperfect contact. Thecomparison between these two measurements will allow the circumferentialapplication of the pad 210 to the wall of the borehole 12 to bemonitored.

The small width of current electrodes 216a and 226 will not renderunacceptable the signal-to-noise ratio of the measurement signalsobtained therefrom because the current density is very large in thelower region of face section 212. Additional lateral electrodes could beprovided in other regions of face section 212 where the current densityis high. For instance, a second lateral current electrode could bedisposed at the level of current electrode 216b, and a third lateralcurrent electrode could be disposed at the level of current electrode216c.

In order to reduce the influence of mud resistivity on the measurementsfurnished by current electrodes 216a and 226, voltage measuringelectrodes 228 and 230 are disposed in the insulating face section 214and respectively associated with the current electrodes 216a and 226.The electrodes 228 and 230 are aligned in the longitudinal direction ofthe pad 210 respectively with current electrodes 216a and 226 and in thelateral center of insulating face section 214.

The voltage electrodes 228 and 230 provide respective voltagemeasurements which allows correction for the influence of mud to beeffected. Assume I_(a) is the current emitted from current electrode216a and V the voltage applied to face section 212 and each of thecurrent electrodes 216a-216g, 224 and 226. The apparent resistivitymeasurement from electrode 216a is:

    R.sub.a =k.sub.a (V/I.sub.a);                              (1)

k_(a) being a constant coefficient. The mud correction comprisessubstituting for this expression the following:

    R.sub.a.sup.* =k.sub.a (V-V.sub.m)/I.sub.a ;               (2)

where V_(m) is the voltage measurement taken from voltage electrode 228.The correction is carried out at least for the very shallow measurementsfrom current electrodes 216a and 226. The correction can be effected forthe measurements from other current electrodes, if desired.

Electronics suitable for operation of the pad 210 in association withsonde 200 are shown generally at 220 in FIG. 11. Signal generator 250,having characteristics identical to signal generator 94, supplieselectric current to face section 212 and to the individual currentelectrodes 216a-216g, 224, and 226 (not shown) through respectivemeasurement signal circuits 252a-252g, 254 and 256 (not shown). Each ofthe measurement signal circuits 252a-252g, 254 and 256 essentially isidentical to measurement signal circuit 96c shown in FIG. 4, with theassociated input transformer and preamplifier preferably mounted insidea pressure-resistant metal tube 260 contained within the body of pad 210(FIG. 12). Pad 210 suitably includes conductive metal lateral edges 261and 263 and a conductive metal back 265, which minimize the influence ofvariation in the distance between sonde 200 and pad 210. Space withinthe body of pad 210 may be filled with hard rubber, epoxy or oil.Current is returned to signal generator 250 via sonde section 206.Additional focussing current is supplied to sonde section 202 and alsoreturned via sonde section 206. The voltage electrode 228 is coupled topreamplifier 258, which furnishes the value of parameter V_(m). Voltageelectrode 260 and associated electronics would be identical, and are notshown in the interest of clarity.

The operating principle of the sonde 200 is illustrated schematically inthe right hand part of FIG. 9. While quite similar in principle to sonde32 (FIG. 2), sonde 200 has a greater depth of investigation. The currentpaths 262 and 264 from the upper region of face section 212 are stronglyfocussed and penetrate deeply into the formation substantially throughthe invaded zone. The focussing is moderate in the central region offace section 212, as indicated by current paths 266, 268, and 270.Current from the lower region of the face section 212 is very weaklyfocussed and has very little penetration, as shown by current path 272.The plurality of apparent resistivity measurements obtained from currentelectrodes 216a-216g and 226, corrected by the measurements obtained byvoltage electrodes 228 and 230, are processed in processor 26substantially as described above, to yield R_(XO) or a resistivityprofile, as desired.

Arm Mechanism

Preferably, the longitudinal positions of the sonde 32 and pad 42 andsonde 200 and pad 210 are to be maintained stable relative to oneanother. An arm mechanism particularly suitable for the various sondeembodiments described herein is shown in FIGS. 14-16. The arm mechanism,which can be accomodated in a relatively short portion of the body of asonde while achieving the desired stability and eccentering by use of along pad-supporting arm, also is suitable for use generally in othertypes of eccentric sondes.

The arrangement of the arm mechanism in the pad extension mode is shownin FIG. 14. The arm 300 at the end of which is mounted pad 302, whichmay be pad 42 or 210, for example, is pivotally connected to the uppersonde body part 304 by a pivot 306. The arm 300 has an integralextension 308 on the end farthest from the pad 302. The extension 308 isresiliently connected to the upper sonde body part 304 by a rod 310passing through a hole 312 formed in a wall portion 314 rigid with thesonde body part 304. The rod 310 carries at one end a pivot 316 and isthus allowed to pivot with respect to the arm 300. At its other end, therod 310 has a stop collar 318 on which bears a compression spring 320,the other end of which engages the wall portion 314 rigid with the upperbody part 304.

The arm 300 is essentially constituted by the body 322 of ansimple-action actuator controlled from the surface equipment. Theactuator is operable to displace an actuating rod 324 projecting fromthe actuator adjacent the pivot 306. The actuator can be a simple-actionhydraulic or electric jack, the rod 324 being secured to the movablemember of the jack. Such devices are well known.

The pad 302 is connected to an end portion 326, which is rigid with theactuator body 322, by means of two links 328 and 330. The upper link 328is articulated on the end portion 326 by a pivot 332 and on the upperend of the pad 302 by a pivot 334. The lower link 330 is articulated bya pivot 336 on the end portion 326, and its end 338 is slidable in anelongate slit 340 formed on the rear of the pad 300. A leaf spring 341is secured on the end portion 326 and connected to approximately themiddle of the rear of pad 302. Spring 341 exerts a resilient force thatacts to engage the pad 302 with the wall of borehole 12. Since the endof the link 330 is slidable with respect to the pad 302, the latter isallowed a limited pivotal displacement about the pivot 334 and hence anangular offset with respect to the axis of the sonde body, so as tooptimally adapt to the profile of the wall of borehole 12. Moreover, theactuator body 322 carries on its outer side a protruding member 342 forengagement with the wall of borehole 12.

The backup arm 344 is pivotally connected to the extension 308 of thearm 300 by a pivot 346 and includes beyond the pivot 346 a forkedextension 348 slidably engaging a peg 350 secured to the actuating rod324. A leaf spring 352 secured to the extension 308 of arm 300 has atits end a roller 354 engaging the internal surface of the backup arm344, so that the backup arm 344 is resiliently biased away from thesonde body.

The lower portion of the sonde body part 304 is designed to accommodatethe pad 302, the arm 300 and the backup arm 344 when retracted, as shownin FIGS. 15 and 16. The lower portion of sonde body part 304 comprisestwo walls 360 and 362 spaced from each other and having inner recessesfor receiving conductors 364.

In the position of FIG. 14 where the pad 302 engages the wall ofborehole 12 for carrying out measurements, the actuator 322 is notoperative and the actuating rod 324 is freely movable with respect tothe actuator body 322. The position of the rod 324 is related to theangular position of the backup arm 344 which is resiliently loaded intoengagement with the borehole wall, and therefore indicative of thediameter of the borehole 12. A signal indicative of this diameter isproduced by a displacement detector (not shown) associated with the rod324, for instance a potentiometer. Due to the resilient connectionbetween the arm 300 and the sonde body provided by the rod 310, thesonde body is subjected to a resilient force that tends to eccentralizethe same with respect to the axis of the borehole 12 and thereby tomaintain the distance between the sonde body and the pad 302 at anessentially constant, low value.

When it is desired to retract the arm 302 and the backup arm 344, theactuator 322 is operated. The actuating rod 324 is displaced to pivotthe backup arm 344 against the action of the spring 352. The backup arm344 pivots inwardly until it abuts a roller 366 mounted at the end of anextension of the lower link 330. Once the backup arm has reached thisabutment position, the continuing displacement of the rod 324 causes thearm 300 to be angularly moved inwardly about pivot 306 until theposition of FIG. 15 is attained.

While the invention has been described with reference to particularembodiments, it is to be appreciated that the embodiments areillustrative and that the invention is not intended to be limited toonly the disclosed embodiments. Variations within the spirit and scopeof the invention will occur to those skilled in the art. Accordingly,variation in these and other such features are contemplated and arewithin the scope of the present invention.

What is claimed is:
 1. An apparatus for providing signals indicative ofthe electrical resistivity of a subsurface formation traversed by aborehole, comprising:an elongate sonde body, said body including a firstelectrically conductive section, an insulating section, and a secondconductive section in endwise connection; a pad adapted for lateralextension from said body into engagement with the wall of the boreholeand having a wall engaging face of conductive material, said conductiveface including a plurality of current electrodes disposed at differentlongitudinal locations along said conductive face and characterized by adecreasing width and an increasing length as their respective distancefrom the edge of said conductive face nearest said second conductivebody section increases; means for maintaining the edge of saidconductive face nearest said second conductive body section in a planetransversely intersecting said sonde body proximate to the juncture ofsaid first conductive body section and said insulating body section;means for maintaining said conductive face and said first conductivebody section at substantially equal electrical potential; and means formaintaining said second conductive body section at a different potentialfor returning current emitted from said first conductive body sectionand said conductive face.
 2. An apparatus as in claim 1 wherein saidcurrent electrodes are centered on the longitudinal axis of saidconductive face.
 3. An apparatus as in claim 1 wherein one of saidcurrent electrodes is located adjacent the edge of said conductive facenearest said second conductive body section.
 4. An apparatus as in claim1 wherein:said plurality of current electrodes is at least three; one ofsaid current electrodes is located adjacent the edge of said conductiveface nearest said second conductive body section; another of saidcurrent electrodes is located within a central area of said conductiveface; and the other of said current electrodes are located between saidedge-located electrode and said center-located electrode.
 5. Anapparatus as in claim 1 wherein said pad further comprises an additionalcurrent electrode located adjacent the edge of said conductive face mostremote from said second conductive body section for determining theapplication of said pad to the borehole wall.
 6. An apparatus as inclaim 1 wherein said pad further comprises an additional currentelectrode located laterally of one of said electrodes for determiningthe application of said pad to the borehole wall.
 7. An apparatus as inclaim 1 wherein:said current electrodes are centered on the longitudinalaxis of said conductive face; and one of said current electrodes islocated adjacent the edge of said conductive face nearest said secondconductive body section;further comprising an additional currentelectrode located laterally said edge-located electrode for determiningthe application of said pad to the borehole wall.
 8. An apparatus as inclaim 1 wherein the edge of said conductive face nearest said secondconductive body section is located in a plane transversely intersectingsaid first conductive body section.
 9. An apparatus as in claim 1wherein said pad futher comprises a face of insulating material adaptedfor engagement with the borehole wall, said insulating face extendingfrom said conductive face toward said second conductive body section andincluding a voltage electrode located therein.
 10. An apparatus as inclaim 9 wherein the edge of said insulating face nearest said secondconductive body section is located in a plane transversely intersectingsaid first conductive body section proximate to the juncture of saidfirst conductive body section and said insulating body section.
 11. Anapparatus as in claim 7 wherein said pad futher comprises a face ofinsulating material adapted for engagement with the borehole wall, saidinsulating face extending from said conductive face toward said secondconductive body section and including a first voltage electrode locatedtherein and centered on the longitudinal axis of said pad in associationwith said edge-located electrode, and a second voltage electrode locatedlaterally of said first voltage electrode in association with saidadditional electrode.
 12. An apparatus as in claim 11 wherein the edgeof said insulating face nearest said second conductive body section islocated in a plane transversely intersecting said first conductive bodysection proximate to the juncture of said first conductive body sectionand said insulating body section.
 13. An apparatus as in claim 1 furthercomprising means for eccentering said sonde body in a direction towardsaid pad.
 14. An apparatus for providing signals indicative of theelectrical resistivity of a subsurface formation traversed by aborehole, comprising:an elongated sonde body comprising:an insulatingbody section; a conductive focussing body section extendinglongitudinally from one end of said insulating body section; and aconductive return body section extending longitudinally from the otherend of said insulating body section; a wall engaging member having awall engaging face comprising:a conductive focussing section; and atleast three longitudinally arranged current electrodes located inrespective areas of said focussing face section having unequal currentdensities and respectively having an increasing longitudinal dimensionand a decreasing transverse dimension with increasing distance from saidreturn body section; and means for urging said wall engaging memberagainst the borehole wall and maintaining said current electrodessubject to a focussing field maintained by said focussing body section.15. An apparatus as in claim 14 wherein said face further comprises anadditional current electrode located adjacent the edge of saidconductive face section most remote from said return body section formeasuring the application of said face to said borehole wall.
 16. Anapparatus as in claim 14 wherein one of said current electrodes isnearest said return body section and adjacent the edge of saidconductive face section.
 17. An apparatus as in claim 16 wherein saidface further comprises an insulating section extending longitudinallyfrom said conductive face section toward said return body section andhaving a potential measure electrode located therein and associated withsaid nearest current electrode for correcting for mud resistivity. 18.An apparatus as in claim 16 wherein said face further comprises anadditional current electrode located adjacent the edge of saidconductive face section and laterally from said nearest currentelectrode for measuring the application of said face to said boreholewall.
 19. An apparatus as in claim 18 wherein said face furthercomprises an insulating section extending longitudinally from saidconductive pad section toward said return body section and having twopotential measure electrode located therein and associated respectivelywith said nearest current electrode and said additional currentelectrode for correcting for mud resistivity.
 20. A wall engaging memberadapted for connection to logging apparatus having a current returnelectrode for providing signals indicative of electrical conductivity ina formation traversed by a borehole, said wall engaging member having awall engaging face comprising:a conductive focussing section; a firstcurrent electrode located in a high current density region of saidfocussing section; a second current electrode located in a low currentdensity region of said focussing section; and a third current electrodelocated in a moderate current density region of said focussing section;the current electrodes being disposed longitudinally one after the otherand the current return electrode being disposed longitudinally on oneside of the set of said current electrodes; wherein with decreasingcurrent density, said current electrodes respectively have an increasinglongitudinal dimension and a decreasing transverse dimension.
 21. Amethod for providing signals indicative of the electrical resistivity ofa subsurface formation traversed by a borehole, comprising the stepsof:emitting survey currents having preselected signal-to-noisecharacteristics from at least three longitudinal locations substantiallyon the wall of the borehole; receiving said survey currents at alocation in the borehole, said longitudinal wall locations being spacedat progressively greater distances from said borehole receivinglocation; emitting additional current from locations substantially onthe wall of the borehole and in the borehole and receiving saidadditional current at said borehole receiving location to differentlyfocus said survey currents and to focus said survey currents essentiallyin a radial plane intersecting the wall locations at which said surveycurrents are emitted; and measuring said survey currents at saidrespective wall locations.